Many systems for generating pseudo three-dimensional (3D) images have been developed over recent years. Generally, these systems can be characterised by the methods by which the observer is deceived into perceiving the image as three-dimensional (i.e. having depth).
In the real-world, the human eye perceives depth in an image due to the combination of a number of visual cues.
With the first visual cue, more distant objects are perceived by the observer to be both smaller and higher in the field of view, than objects that are closer to the observer. Typically, distant objects are also blocked from the observer's field of view by closer objects and the observer perceives the resolution, contrast, and brightness to be less well defined.
With the second visual cue the observer perceives an apparent change in the position of the object relative to the more distant background image as his own position changes. This effect is known as parallax and can effect the image perceived by the observer in both the horizontal and vertical planes.
With the third visual cue the lateral separation of the observer's eyes means that the distance between a point on an object and each eye can be different. This effect is known in the art as binocular disparity and results in each eye observing a slightly different perspective. However, in real-life, this effect is resolved by the human brain to produce the single clear image perceived by the observer.
The fourth visual cue to three-dimensional perception of video images is depth disparity. Since the human eye has a finite field of view in both the horizontal and vertical planes, the eye tends to focus on an object, or region of an object, that is of immediate interest. Consequently, surrounding objects, or regions of the object, which form the background image are out of focus and blurred. The human brain perceives these surrounding objects or regions to be at a different distance to provide a depth cue.
Known stereoscopic and auto-stereoscopic systems for generating pseudo three-dimensional images, generate alternate and slightly differing frames of the video image for each eye. The different frames are intended to correspond to the different views perceived by the human brain due to the separation between the eyes, and produce a binocular disparity.
The observer of a video image generated using a stereoscopic system must be provided with an optical device such as a pair of spectacles having one red lens and one green lens. A separate frame of the video image is shown alternately for each eye and at sufficient frequency that the observer resolves a single image.
Auto-stereoscopic systems were developed to produce video images with multiple image planes (i.e. the observer can view around foreground objects). These auto-stereoscopic systems are designed to focus separate frames of the image into each eye using an arrangement of optical elements. Typically, these elements will include vertically aligned lenticular lenses. These systems have found application in items such as postcards, but their more widespread use is limited by the narrow field of view.
As the observer of a stereoscopic or auto-stereoscopic image changes their point of focus, either by looking from one object to another, or by looking at a different region of the object, the eyes must readjust. Each eye will take a finite period to adjust to the focal plane associated with the object perceived by the observer. Therefore, the focal plane of the image perceived by each eye may differ and the human brain must converge the images into a single focused image of the object (known in the art as Convergence).
Similarly, the human eye has a finite depth of focus, or region in space in which the focus of an object can be resolved. This is due to the physical requirement for the cornea to change shape to produce a sharp image of the object on the surface of the retina. Therefore, as the observer switches his attention from a distant object to a close object or vice versa, objects outside the field of view become less well defined and blur (known in the art as Accommodation).
Recent research has shown that users of stereoscopic and auto-stereoscopic systems are prone to fatigue, eye-strain, and headaches. It is thought that this can be attributed to the fact that convergence and accommodation of images in the real-world coincide, and hence the human brain interprets muscular actions associated with the control of the cornea to predict that objects are at different distances.
Conversely, in stereoscopic and auto-stereoscopic systems convergence and accommodation occur at different points in space. FIG. 1 illustrates a stereoscopic system for generating three-dimensional video images in which a display screen 10, such as an LCD or CRT display, shows an image 12 of an object. The eyes of the observer 16 are focused on the display 10 producing an accommodation distance Da. However, the object 12 is perceived to be in front of the display 10, and hence the convergence distance Dc at which the image 14 of the object 12 is perceived is between the display 10 (where the object is in focus) and the observer 16.
Since the object 12 is not perceived by the observer 16 to be at the display surface 10, the human brain directs the eyes at the point in space where it predicts the image 14 to be. This results in the human brain being provided with conflicting signals that are indicative of the accommodation and convergence and can result in fatigue, eye strain and headaches.